1. Field of the Disclosure
The present disclosure relates to masking one or more scan chains in a testing circuit for testing an integrated circuit to prevent propagation of unknown values.
2. Description of the Related Art
A defect is an error introduced into an integrated circuit (IC) during a semiconductor manufacturing process. Defects that alter the behavior of the IC can be described by a mathematical fault model. During testing of the IC, a test pattern is applied to the IC and logic value outputs from the IC are observed. When the IC is operating as designed, the logic value output coincides with expected output values specified in test patterns. A fault in the IC is detected when the logic value output is different than the expected output.
Automatic Test Pattern Generation (ATPG) refers to an electronic design automation (EDA) process that generates a set of test patterns for applying to an IC to detect faulty behavior caused by defects in the IC. The generated patterns are used to test semiconductor devices after manufacture, and in some cases to assist with determining the cause of fault. The fault model may be used to generate the test patterns that effectively covers certain types of faults with a fewer number of test patterns.
To receive and detect faults in the IC, the IC includes a test circuit that receives and applies the test patterns to one or more scan chains. A scan chain includes a row of multiple scan flops that output a certain logic value when the test pattern is applied. An unexpected output of a scan flop is indicative of certain faults or defects in circuit components associated with the scan flop.
Outputs of multiple scan flops may be compressed into a bit stream to reduce data bandwidth and pins associated with the testing of the IC. Compression of the output of the scan flop into a bit stream decreases the amount of information that may be extracted from the bit stream. For instance, an unexpected value on one of the bits of the bit stream may be associated with multiple scan flops and determination as to which scan flop caused the unexpected value may not be easily made.
An unknown value in the output of a scan chain is designated “X” to represent that it could be either a logic 0 or logic 1 in the expected scan-out data stream. X response values in output of scan chains are undesirable because it is unknown whether the design is faulty based on the scan output. In standard scan testing without compression, X values are simply ignored. However, when scan data is compressed, X values can interfere with known values and reduce fault coverage. Therefore, compression of scan output can reduce observability and worsen test coverage.